Wippermann makes THIS chainring wear indicator. I have one, and it seems to work. Here are some images of sprockets; I will try to get some of chainrings, but they should be similar.


Here is the gauge showing a gap between the tool and the base of the notch... a good sprocket (new, actually).



Here is a worn-out sprocket.



This is an un-used sprocket from a cassette. Gap indicates "good" (of course).